Microelectromechanical system having movable element integrated into substrate-based package

ABSTRACT

A semiconductor-centered MEMS device ( 100 ) integrates the movable microelectromechanical parts, such as mechanical elements, flexible membranes, and sensors, with the low-cost device package, and leaving only the electronics and signal-processing parts in the integrated circuitry of the semiconductor chip. The package is substrate-based and has an opening through the thickness of the substrate. Substrate materials include polymer tapes with attached metal foil, and polymer-based and ceramic-based multi-metal-layer dielectric composites with attached metal foil. The movable part is formed from the metal foil attached to a substrate surface and extends at least partially across the opening. The chip is flip-assembled to span at least partially across the membrane, and is separated from the membrane by a gap.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 12/701,683filed Feb. 8, 2010 (now U.S. Pat. No. 8,304,274), the entirety of whichis incorporated by reference herein, which claims the benefit ofProvisional Application Nos. 61/291,767 filed Dec. 31, 2009, and61/152,607 filed Feb. 13, 2009.

FIELD OF THE INVENTION

This relates in general to the field of semiconductor devices andprocesses, and more specifically to the structure and fabrication ofmicroelectromechanical system (MEMS) devices having a movable elementintegrated into a substrate-based ball grid array package and a sensingelement built on an integrated circuit.

DESCRIPTION OF RELATED ART

The wide variety of products collectively called microelectromechanicalsystem (MEMS) devices are small, low weight devices on the micrometer tomillimeter scale produced on the basis of batch fabrication techniquessimilar to those used for semiconductor microelectronics devices. MEMSdevices integrate mechanical elements, sensors, actuators, andelectronics on a common carrier. MEMS devices have been developed tosense mechanical, thermal, chemical, radiant, magnetic, and biologicalquantities and inputs, and produce signals as outputs.

MEMS devices may have parts moving mechanically under the influence ofan energy flow (acoustic, thermal, or optical), a temperature or voltagedifference, or an external force or torque. Certain MEMS devices with amembrane, plate or beam can be used as a pressure sensor or actuator(for instance microphone and speaker), inertial sensor (for instanceaccelerometer), or capacitive sensor (for instance strain gauge and RFswitch); other MEMS devices operate as movement sensors for displacementor tilt; bimetal membranes work as temperature sensors. Besides smallsize, the general requirements for the membrane- or plate-operatedsensors include long term stability, small temperature sensitivity, lowhysteresis for pressure and temperature, resistance to corrosiveenvironments, and often hermeticity.

In a MEMS device, the mechanically moving parts are fabricated togetherwith the sensors and actuators in the process flow of the electronicintegrated circuit (IC) on a semiconductor chip. As an example, themechanically moving parts may be produced by an undercutting etch atsome step during the IC fabrication. Bulk micromachining processesemployed in MEMS device sensor production for creating, in bulksemiconductor crystals, the movable elements and the cavities for theirmovements include anisotropic wet etching, reactive ion etching (RIE),and deep reactive ion etching (DRIE). These techniques employphotolithographic masking, are dependent on crystalline orientation, andneed etch stops, all of which are expensive in terms of time andthroughput. In addition, there are bulk and surface micromachiningtechniques for building up structures in thin films on the surface ofsemiconductor wafers, also expensive techniques. While many of theprocesses are expensive to implement, some processes, such as automaticwafer bonding, are inexpensive.

Because of the moving and sensitive parts, MEMS devices have a need forphysical and atmospheric protection. Consequently, MEMS devices aresurrounded by a housing or package, which has to shield the MEMS deviceagainst ambient and electrical disturbances, and against stress. Formany devices, fully hermetic and even quasi-hermetic packages representa significant cost adder, especially when ceramic packages or precisionparts such as glass plates are required.

Among the basic operating principles of pressure sensors arepiezoresistive, capacitive, and resonant operation. In thepiezoresistive operation, the pressure is converted to an electronicallydetectable signal, wherein the conversion relies on the elasticdeformation of a structure such as a membrane exposed to the pressure;pressure causes strain, and strain causes change of electricalresistivity. In MEMS device silicon technology, controlling the membranethickness, size, and alignment involves precision process steps. In theresonant operation, the pressure causes mechanical stress in thevibrating microstructure; the resonance frequency is measured independence on the mechanical stress. Excitation and damping of the MEMSdevice silicon diaphragm and the nonlinear frequency-pressurerelationship require sophisticated calibration. In the capacitiveoperation, the pressure causes a displacement-dependent output signal.The change in pressure causes a displacement, the displacement causes acapacitor change, and the capacitor change causes an electrical signalsimilar to the operation of a condenser microphone. Nonlinearity andparasitic capacitances and residual membrane stress represent challengesfor MEMS device membrane fabrication of silicon and epitaxial silicon.

Taking the example of capacitive pressure sensors, several fabricationmethods may be chosen. In one method, the sensors are bulkmicro-machined as a glass-silicon-glass structure with verticalfeed-throughs. In another method, a preferentially etched wafer receivesdeep and shallow boron diffusions and dielectric depositions, which aremounted on glass so that the wafer can finally be dissolved. In yetanother method, a surface micro-machined capacitive pressure sensor iscreated by a polysilicon layer (1.5 μm thick) separated by a gap (0.8 μmwide) over the n+ doped silicon electrode; the sensor is monolithicallyintegrated with the sensing circuitry. The sensors are small and span anoperating range from about 1 bar to 350 bar, have high overpressurestability, low temperature dependence and low power consumption.

In the basic operating principle of accelerometers, the mechanical andelectrical sensitivity are a function of the vertical displacement ofthe movable plate's center. In displacement sensing accelerometers, theapplied acceleration as input is transformed into the displacement ofthe movable mass (plate) as output; a suspension beam serves as theelastic spring. Force sensing accelerometers detect directly the forceapplied on a proof mass. The MEMS device fabrication in bulksingle-crystal silicon of the movable plate, the suspension beam, andthe proof mass requires sensitive semiconductor etching techniques.

SUMMARY

Applicants believe manufacturing cost is the dominant factor preventingthe widespread integration of pressure sensors, microphones,accelerometers and other applications in which a movable member isneeded to convert an external analog input into an electrical output,into systems in the automotive, medical, and aerospace industries.

Applicants saw that building a MEMS device on the surface or within thewafer by standard wafer fab technology and standard wafer fablithographic methods is not only a high cost approach, but also limitsthe choice of materials and configuration available for the MEMS devicecomponents, which have to be compatible with the standard wafer process.After the wafer fabrication, in standard technology, the MEMS devicesstill have to be packaged using known packaging material andprocesses—another cost adder.

Applicants solved the problem of mass-producing low costsemiconductor-centered MEMS devices by integrating the movable MEMSdevice parts, such as mechanical elements and sensors, including theircomplete fabrication with low-cost device materials and packages, and byleaving only the electronics and signal-processing parts in theintegrated circuitry. The package, into which the movable parts areintegrated, may either be a leadframe-based or a substrate-based plasticmolded housing. With this invention, the MEMS devices may use a standardCMOS chip without any movable structure and a packaging component withmovable structures built therein.

Applicants further discovered that the separation of movable andelectronics parts provides greater system level integration with othercomponents such as package-on-package MEMS devices, thus increasing theelectrical product efficiency.

In embodiments, which have the movable elements integrated into asubstrate-based package, the substrate may be a stiff multi-layersubstrate, such a multi-metal-layer FR-4 board, or a flexible filmsubstrate, such as a metalized polyimide tape. The latter devices need amolded encapsulation for robustness. Packages can be stacked with solderbodies as connecting elements.

Embodiments of this invention include the usage of electrostatic force,acceleration, air pressure, etc., to deflect a beam or membrane forbuilding microphones, pressure sensors, accelerometers, and otherapplications where a movable member is needed to convert an externalinput into an electrical output.

Example MEMS devices of the pressure sensor family, operating oncapacitive changes caused by a movable membrane, may offer 80% lowerfabrication cost, when the membrane is integrated into the plasticdevice package instead of being fabricated in conventional manner as aportion of the silicon chip.

One embodiment of the present invention provides a MEMS devicecomprising: a flat substrate having a thickness, a first surface and anopposite second surface; an opening through the thickness of thesubstrate, the opening extending from the first to the second surface; ametal foil attached onto the first surface of the substrate, the foilincluding a plurality of pads and a membrane extending at leastpartially across the opening; and an integrated circuit chipflip-assembled to the pads, the chip at least partially spanning acrossthe opening, separated from the membrane by a gap.

Another embodiment of the present invention provides a method forfabricating a MEMS device comprising the steps of: forming an openingfrom a first surface to an opposite second surface of a flat substrate;laminating a metal foil onto the first substrate surface and at leastpartially across the opening so that the foil adheres to the substrate;patterning the metal layer into a plurality of pads and a segment; andflip-connecting a semiconductor chip having electronic circuitry ontothe pads so that the chip spans across at least partially across theopening, separated from the segment by a gap.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic cross section of an example MEMS deviceof the inertial sensor family operating as a capacitive accelerometer,where the movable membrane is part of the substrate-based package. Inthe example shown, the substrate is a multi-metal layer insulatingcomposite, and all pads and the membrane are on one surface of thecomposite in the same plane.

FIG. 2 shows a schematic cross section of an example MEMS device of theinertial sensor family operating as a capacitive accelerometer, wherethe movable membrane is part of the substrate-based package. In theexample shown, the substrate is an insulating tape and the metal padsand the membrane are on one surface of the tape in the same plane; anencapsulation may be added to enhance the package robustness.

FIG. 3 shows a schematic cross section of an example MEMS device of theinertial sensor family operating as a capacitive accelerometer, wherethe movable membrane is part of the substrate-based package. In theexample shown, the substrate is an insulating tape, the membrane andsome pads are on one surface of the tape, other pads on the oppositesurface; an encapsulation may be added to enhance the packagerobustness.

FIG. 4A depicts a schematic cross section of an example MEMS device ofthe pressure sensor family operating in the capacitive mode, where themovable membrane is part of the substrate-based package. In the exampleshown, the substrate is an insulating tape; an encapsulation may beadded to enhance the package robustness.

FIG. 4B shows a top view of an example outline of the membrane in FIG.4A.

FIG. 5A depicts a schematic cross section of another example MEMS deviceof the inertial sensor family operating as a capacitive accelerometer,where the movable membrane is part of the substrate-based package.

FIG. 5B is a schematic top view of a membrane for the MEMS device inFIG. 5A including lateral sensing.

FIG. 5C is a schematic top view of a fingered membrane for the MEMSdevice in FIG. 5A to increase the sensitivity for lateral movement.

FIG. 5D is a schematic top view of a symmetrically balanced membrane forthe MEMS device in FIG. 5A for sensing rotational acceleration.

FIG. 6 shows a schematic cross section of a hermetic MEMS deviceaccelerometer, where the electrostatically lifted proof mass is part ofthe substrate-based package.

FIG. 7 depicts a schematic cross section of a hermetic MEMS deviceaccelerometer with a symmetrically balanced proof mass.

FIG. 8 illustrates a schematic cross section of a hermetic MEMS deviceaccelerometer with a semiconductor chip attached on only one side toallow an enlarged proof mass sensor.

FIGS. 9A to 9J illustrate certain process steps of a fabrication flowfor an inertial sensor MEMS device with the movable membrane integratedinto the substrate-based package.

FIG. 9A is a schematic cross section of the dielectric substrate withopenings formed form the first to the second substrate surface.

FIG. 9B is a schematic cross section of the substrate after depositing alayer of adhesive material on the first surface of the carrier.

FIG. 9C is a schematic cross section of the substrate after laminating ametal foil across the adhesive layer.

FIG. 9D is a schematic cross section of the substrate after depositing aphotoresist layer across the metal foil and another photoresist layeracross the contoured second substrate surface.

FIG. 9E is a schematic cross section of the substrate after patterningthe metal foil and removing both photoresist layers.

FIG. 9F is a schematic cross section of the substrate after the processstep of flip-connecting semiconductor chips.

FIG. 9G is a schematic cross section of the assembled substrate afterattaching solder balls for external connection.

FIG. 9H shows a schematic cross section of singulated completed inertialsensor MEMS device with the movable membrane integrated into thesubstrate-based package.

FIG. 9I is a schematic cross section of the assembled substrate afterdepositing an encapsulation compound.

FIG. 9J shows a schematic cross section of singulated completed inertialsensor MEMS device with the movable membrane integrated into thesubstrate-based package strengthened by encapsulation compound.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1 illustrates a schematic cross section of an example embodiment ofthe invention showing a microelectromechanical system (MEMS) device ofthe inertial sensor family, which operates as a capacitive accelerometerwith displacement-dependent output signals. In these sensors,acceleration is transformed into the displacement of a movable mass orplate; the position change is measured as a change of the capacitancerelative to a fixed plate. Capacitive accelerometers exhibit highsensitivity, good DC response and noise performance, low drift, and lowpower dissipation and temperature sensitivity. The example MEMS device,generally designated 100 in FIG. 1, is a system structured like QFN(Quad Flat No-lead) and SON (Small Outline No-Lead) type semiconductordevices.

In the embodiment of FIG. 1, an integrated circuit device, representedby chip 101, is flip-assembled on metallic pads 110. Flip-assemblytypically utilizes conductive spacers such as solder balls or solderbodies to mechanically and electrically attach a chip surface, on whichan integrated circuit has been formed, to an opposing surface of asubstrate which interconnects multiple integrated circuits or otherelectrical components. Metallic pads 110, which are attached to thefirst surface 120 a of a substrate 120. This substrate is flat, has athickness 121, a first surface 120 a and an opposite second surface 120b. Substrate 120 is made of insulating material, which may be apolymeric compound, in some devices strengthened by glass fibers, or aceramic compound, or a glass compound. As examples, substrate 120 may bea sheet or board made of a multi-metal layer composite, as schematicallyindicated in FIG. 1, or it may be a sheet-like multi-metal layer ceramiccomposite. The typical thickness range is between about 70 and 150 μm.Alternatively, the substrate may be a flexible polymeric tape such aspolyimide (as indicated by designation 220 in FIG. 2).

MEMS device 100 has an opening 122 through the thickness 121 ofsubstrate 120. In FIG. 1, the opening has a uniform width 122 a; inother embodiments, the width of the opening may not be constant.Openings 122 may be shaped as a cylinder, a truncated cone, or any othersuitable stereometrical form. As FIG. 1 shows, opening 122 extends fromthe first substrate surface 120 a to the second substrate surface 120 b.Flip-assembled chip 101 spans at least partially across the opening.

As FIG. 1 illustrates, metallic pad 110 is a portion of a patternedmetal foil attached onto the first surface 120 a of substrate 120; anadhesive layer 130 may be used for the attachment (the processes forattaching, such as laminating, and patterning are described in FIGS. 9Ato 9J). The metal foil may be made of copper or a copper alloy; otheroptions include nickel, or an alloy containing an iron-nickel alloy(such as Alloy 42 or Invar™), or aluminum. Preferred foil thicknessrange is between about 5 and 50 μm, more preferably between 10 and 25μm, but may be thicker or thinner in other embodiments. Other portionsof the patterned metal foil include a plurality of pads 111 forconnection to external parts, often using solder bodies 140, and amembrane 112 as a movable part. FIG. 1 depicts membrane 112 extending atleast partially across the opening 122, and being parallel to chip 101.Membrane 112 is separated from chip 101 by gap 107, which has a height107 a about 10 to 60 μm, typically about 25 μm. Acceleration istransformed into the displacement of the movable membrane, and theposition change is measured as a change of the capacitance relative tothe fixed metal layer 108 on chip 101.

Since pads 110 and 111, and membrane 112 are portions of the metal foilattached to substrate surface 120 a, they have the same thickness 113;for many embodiments, thickness typically is between 10 and 25 μm. Inthis thickness range, membrane 112 is flexible in the direction normalto the first substrate surface and movable in the space of the opening122 and of the gap 107.

The example of FIG. 1 shows a simplified, low cost version, where theopening 122 is not sealed but open; sealed embodiments are illustratedin FIGS. 4A and 5A. In all these embodiments, substrate 120 togetherwith the attached patterned metal foil represent the package for chip101; membrane 112 as the movable part of the MEMS device is a portion ofthe package. Examples of the shape of movable part 112 are discussedbelow in FIGS. 5A to 8.

In the example embodiment of FIG. 2, the MEMS device has a substrate 220made of a polymeric tape (for instance polyimide), often in thethickness range from about 60 to 100 μm. In this thickness range,substrate 220 is flexible; consequently, in order to have a mechanicallyrobust MEMS device, many applications require the substrate to bestrengthened by a body 250 of hardened plastic compound such aspolymerized epoxy-based molding compound. The compounds may includeinorganic filler particles (such as silicon dioxide or silicon nitride)of about 80 to 90% by volume in order to better match the coefficient ofthermal expansion (CTE) of the compound to the CTE of silicon. Since theapplication of the encapsulation compound 250 is optional, it is shownin dashed outline in FIG. 2.

As FIG. 2 shows, tape 220 has an opening 222 through the thickness ofthe tape. In FIG. 2, the opening has a uniform width 222 a; in otherembodiments, the width of the opening may not be constant. In addition,the strengthening body 250 has an opening 252 through the bodythickness. Opening 252 feeds into opening 222. Opening 252 may be shapedas a cylinder, a truncated cone wider on the outside and narrowingtowards width 222 a, as depicted in FIG. 2, or any other suitablestereometrical form. Flip-assembled chip 101 spans at least partiallyacross the opening 222.

The MEMS device of FIG. 2 has, in analogy to the device in FIG. 1, apatterned metallic foil attached to the first surface 220 a of the tape220. An adhesive layer 230 may be used for the attachment (the processesfor attaching, such as laminating, and patterning are described in FIGS.9A to 9J). Typically the metal foil is made of copper or a copper alloy;other options include nickel, or an alloy containing an iron-nickelalloy (such as Alloy 42 or Invar™), or aluminum. The foil thicknessrange is between about 5 and 50 μm, typically between about 10 and 25μm, but may be thicker or thinner in other embodiments. The metal foilincludes an elongated portion 212, which is operable as a movable partor membrane. Other portions of the patterned metal foil include aplurality of pads 211 for connection to external parts, often usingsolder bodies 240, and pads 210 for contacts to semiconductor chip 101.FIG. 2 depicts membrane 212 extending at least partially across theopening 222, and being parallel to chip 201. Membrane 212 is separatedfrom chip 201 by gap 207, which has a height 207 a of about 10 to 60 μm,typically about 25 μm. Acceleration is transformed into the displacementof the movable membrane, and the position change is measured as a changeof the capacitance relative to the fixed metal layer 108 on chip 101.

Since pads 210 and 211, and membrane 212 are portions of the metal foilattached to substrate surface 220 a, they have the same thickness 213;for many embodiments, thickness 213 is between about 5 and 50 μm,typically between 10 and 25 μm, but may be thicker or thinner in otherembodiments. In this thickness range, membrane 212 is flexible in thedirection normal to the first substrate surface and movable in the spaceof the opening 222 and of the gap 207.

Another example embodiment of the invention, generally designated 300,is illustrated in FIG. 3. The embodiment is a substrate-based MEMSdevice of the inertial sensor family acting as a capacitiveaccelerometer with displacement-dependent output signals. Substrate 320is made of a polymeric tape (for instance polyimide), typically in thethickness range from about 60 to 100 μm, with first surface 320 a andopposite second surface 320 b [note the difference of surfacedesignations compared to FIGS. 1 and 2]. Tape 320 has a plurality ofmetal-filled via holes extending from first surface 320 a to secondsurface 320 b. One set of vias, designated 311, operates as attachmentsites for solder balls 340 to external parts, another set of vias,designated 313, operates as sites for flip-attaching chip 101.

Substrate 320 further has an opening 322 through the thickness ofsubstrate 320, extending from first substrate surface 320 a to secondsubstrate surface 320 b. In FIG. 3, the opening has a uniform width 322a; in other embodiments, the width of the opening may not be constant.Openings 122 may be shaped as a cylinder, a truncated cone, or any othersuitable stereometrical form. Flip-assembled chip 101 spans at leastpartially across the opening, forming a gap 307 of height 307 a withsecond substrate surface 320 b. Height 307 a typically is between about10 and 60 μm, often about 25 μm.

A metal foil 310 is attached onto first substrate surface 320 a,typically using an adhesive layer 330 to support the attachment. Themetal foil may be made of copper or a copper alloy; other optionsinclude nickel, or an alloy containing an iron-nickel alloy (such asAlloy 42 or Invar™), or aluminum. The preferred foil thickness isbetween 10 and 25 μm, but may be thicker or thinner in otherembodiments. The foil is patterned to form a membrane 312 without theadhesive layer. In the foil thickness range indicated, membrane 312 isflexible in the direction normal to the first substrate surface 320 aand movable in the space of the opening 322 and of the gap 307.Acceleration is transformed into the displacement of the movablemembrane 312, and the position change is measured as a change of thecapacitance relative to the fixed metal layer 108 on chip 101.

Since the stack of tape 320, adhesive layer 330, and metal foil 310 hasa total thickness in the range from approximately 70 to 150 μm, it isflexible. When some applications require a mechanically more robust MEMSdevice, the stack may be strengthened by adding a body 350 of hardenedplastic compound such as a polymerized epoxy-based molding compound,optionally filled with inorganic particles of silicon dioxide or siliconnitride. The strengthening body 350 has an opening 352 through the bodythickness. Opening 352 feeds into opening 322 so that is allows theunobstructed operation of membrane 312 in moving in the z-direction.Opening 352 may be shaped as a cylinder, a truncated cone wider on theoutside and narrowing towards width 322 a, as depicted in FIG. 3, or anyother suitable stereometrical form.

Flip-assembled chip 101 spans at least partially across the opening 322so that metal plate 108 forms a capacitor with membrane 312. Plate 108is separated from membrane 312 by a distance, which is composed of thesum of gap height 307 a, the thickness of tape 320, and the thickness ofadhesive layer 330. As stated above, the movable part, membrane 312, canmove in this distance in the z-direction, normal to the plane of themembrane. In some embodiments, movable part 312 includes the suspensionbeam of length 315 and the movable plate of length 316. Movable plate316 has an area equal to the area of the fixed plate 108 on the chipsurface in order to form a capacitor. In addition, for some embodimentsthe mass of the movable plate 316 can be enlarged by adding the mass ofa deformed gold sphere 314, as formed in the well-known wire ball bondprocess. Mass 314 represents the proof mass.

The invention allows the selection of the materials and dimensions foropening 322 (and 352), length of suspension beam 315, area of movableplate 316, mass 314, and capacitance between movable plate 316 and fixedplate 108. Consequently, the accelerometer of FIG. 3 can be specializednot only as a capacitive displacement sensing accelerometer, whichtransforms acceleration into the displacement of a movable mass, butalso as a force sensing accelerometer, which detects directly the forceapplied on a proof mass. The mechanical transfer function of theselected components relates applied acceleration as the input to thedisplacement of the mass (movable plate 312 and mass 314) as the output.The components of FIG. 3 allow a designed distribution of the outputbetween the additive forces: inertial force, elastic force, and dampingforce.

The example embodiments shown in FIGS. 4A, 5A, 6, 7, and 8 illustrateMEMS device with enclosures, which are quasi-hermetic in polymericpackages, and fully hermetic in packages using ceramic or glassyencapsulations. The embodiment of FIG. 4A displays a QFN/SON-type MEMSdevice, generally designated 400, of the pressure sensor family, whichoperates in the capacitive mode with displacement-dependent outputsignals. An integrated circuit chip 401 is flip-assembled onto metallicpads on the surface of a substrate 420. The substrate may be amulti-metal layer insulating composite, as discussed above in FIG. 1, ora polymeric sheet, as discussed in FIG. 2, or a polymeric sheet withmetal-filled via holes, as discussed in FIG. 3. Substrate 420 in FIG. 4Aillustrates the latter embodiment. The metal-filled vias are designated421. The use of solder balls 440 for interconnection makes these devicesespecially suitable for high input-output counts, multi-chip modules andpackage-on-package modules.

Substrate 420 has an opening 422 through the thickness of the substrate,extending from first substrate surface 420 a to second substrate surface420 b. In FIG. 4A, the opening has a uniform width; in otherembodiments, the width of the opening may not be constant. The openingsmay be shaped as a cylinder, a truncated cone, or any other suitablestereometrical form. The flip-assembled chip 401 spans at leastpartially across the opening, forming a gap 407 with first substratesurface 420 a. The gap has a typical height between about 10 and 60 μm,more typically about 25 μm.

Substrate 420 has a patterned metal foil attached to its first substrate420 a; an optional adhesive attachment layer is not shown in FIG. 4A.The foil typically is made of metal such as copper or nickel,alternatively of an iron-nickel alloy (such as Alloy 42 or Invar™) or ofaluminum. For many embodiments, the thickness is between about 5 and 50μm, preferably between about 10 and 25 μm, but may be thicker or thinnerin other embodiments. The pattern of the foil includes at least onemovable part and a plurality of pads; in general, the pattern allowscomplicated routings of signals. In the thickness range quoted, themovable part 412 can act as a membrane, which is flexible in thez-direction, movable in the space of the opening 422 and of the gap 407.As a membrane, part 412 is sensitive to external pressure changesarriving from z-direction through opening 422, bending the membraneinward and outward of gap 407. In some embodiments, movable part 412 hasan area between about 0.5 and 2.3 mm²; in other embodiments, the areamay be smaller or larger. In some MEMS devices, the membrane may bedivided in a movable plate 412 a and suspension beams 412 b, which holdthe plate. Plate 412 a may have a rectangular or a rounded outline; anexample is illustrated in the top view of FIG. 4B. The suspension beams412 b may take a wide variety of configurations (angular, spring-like,rounded, etc.) to enhance the pressure sensitivity; FIG. 4B depicts anangular configuration. The movable membrane 412 is facing metal plate408 on chip 401 to form a capacitor across gap 407.

For the example embodiment of FIG. 4A, the plurality of pads may begrouped in sets. The pads of the first set, designated 410, enableelectrical interconnection between the movable part 412 and theintegrated circuit of chip 401. The leads of the second set, designated413, enable contacts to external parts; they allow the attachment ofsolder balls 440. The leads of the third set, designated 411, areconfigured as a metal seal ring encircling the opening 422. In general,patterning of the foil enables high density interconnects as well ascomplicated routing of signals.

For the substrate pad sets 410 and 413, a plurality of chip terminals402 allow the connections to solder bodies, gold bumps, or gold alloy.The gold bumps may be produced by a wire ball bonding technique,followed by a flattening process with a coining technique. The goldalloy may be a low melting gold/germanium eutectic with 12.5 weight % Geand an eutectic temperature of 361° C. In addition, chip terminal 403may be configured as a seal ring to allow the formation of a seal ringmade of solder or gold alloy to seal the enclosed space at leastquasi-hermetically, e.g. against environmental disturbances such asparticles, but not completely against gaseous and moisture molecules.FIG. 4A shows the optional encapsulation 450 material filling the spacebetween the chip 401 and substrate 420 up to the seal ring between pad411 and chip terminal 403. The encapsulation 450 typically is fabricatedby a molding technique (for instance transfer molding) using anepoxy-based molding compound; the compound is hardened by polymerizationto give mechanical strength to device 400. The compound may includeinorganic filler particles (such as silicon dioxide or silicon nitride)of about 80 to 90 volume % in order to lower the coefficient of thermalexpansion (CTE) close to the silicon CTE.

Sensing plate 408 and membrane 412, typically having the same area andbeing separated by a gap, form a capacitor. As stated above, membrane412 is made of a metal (for example, copper) thin enough (for example,10 μm) to be flexible and sensitive to pressure changes. The assembleddevice 400, therefore, works as a MEMS device for pressure sensor andmicrophone. Responding to pressure arriving through opening 422 bybending inward and outward, membrane 412 modifies distance 407 relativeto stationary plate 408. Let the area of membrane 412, and plate 408, aselectrodes be A; the distance between the electrodes under originalpressure be D_(o); and the dielectric constant of the space between theelectrodes be ∈, then the capacitance C of the electrodes is given byC=∈·A/D _(o).Pressure in z-direction deforms the flexible membrane so that thedeformed area has to be calculated as an integral over small areaelements dx·dy, while the distance D_(o) is modified in both x-directionand y-direction by a deflection w_(x,y). The resulting change ofcapacitance is measured by the circuitry of chip 401, operating as amicrophone or a pressure sensor. A miniature speaker can be built in asimilar way by driving the membrane electrostatically.

It should be stressed that the embodiments shown in FIGS. 1, 2, 3, and4A lend themselves to stacking of devices; these stacked devices includemulti-chip MEMS devices as well as package-on-package MEMS devices.

In order to give a cost estimate for the example pressure sensor MEMSdevice, the side lengths of the molded material 450 in FIG. 4A may be 3by 3 mm, 4 by 4 mm, 3 by 4 mm, or any other size desired by customers.The base material of the substrate may be polyimide, and the metal foil,including the membrane 412, may be copper. The cost of the moldedpackage, including the movable part, in mass production is about $0.13.With the added cost of the chip about $0.009, the total cost of the MEMSdevice in a plastic package including the movable part according to theinvention is about $0.139. This cost compares to the cost of aconventional pressure sensor MEMS device of the same body sizes and aFR-4 based substrate material as follows: The cost of the conventionalpackage is about $0.54; the cost of the chip including the movable partis about $0.017; the total cost of the MEMS device about $0.557. Thiscost is approximately four-fold the cost of the MEMS device according tothe invention.

FIGS. 5A to 5D illustrate a MEMS device embodiment as a variablecapacitor and RF switch, with the possibility for a fully hermeticpackage, suitable for cellular handset antenna tuning. The constructionof the example MEMS device in FIG. 5A is similar to the device describedin FIG. 1 with the following additions. The opening 522 of substrate 520(shown as a multi-level metal insulating compound or ceramic) is sealedby a lid 550 in flat contact with substrate 520. The lid material may beplastic for quasi-hermetic sealing, or, if fully hermetic sealing isneeded for a ceramic substrate, a moisture-impenetrable material such asglass, ceramic, or, in some devices, metal. Substrate pad 511 andcontact 503 of chip 501 are configured as a seal rings to allow completesealing by solder or a low-melting gold alloy. Membrane 512 as themovable part extends at least partially across opening 522. Membrane 512is shown in FIG. 5A as membrane-at-rest in solid contour, and asdeflected membrane in dashed contour. When deflected, the membrane restson dielectric film 508 a attached to chip plate 508. In this position,the membrane forms a high capacitance with the plate, representing a lowimpedance for RF frequencies; the RF switch is turned on.

The concept to integrate the movable part of a MEMS device with asubstrate-based package rather than with the chip allows numerousvariations with the goal to sensitize certain aspects of themeasurement, or to include new aspects into the measurement. FIGS. 5B,5C, 5D, 6, 7, and 8 highlight only a few select variations andpossibilities and are not intended to be construed in a limiting sense.The figures should emphasize the great number of possibilities of theinvention apparent to persons skilled in the art.

FIGS. 5B, 5C. and 5D show some examples of membrane configurations inleaf spring patterns in order to reduce electrostatic forcerequirements, or to add lateral movement sensitivity. In FIG. 5B, themembrane is divided into a moving portion 512 a and a fixed portion 512b; the design helps to minimize the risk of membrane sticking to theplate on the chip. In FIG. 5C, the design of fingered membranes 512 c,512 d, and 512 e surrounded by fixed metal portions 560 increaseslateral motion and the sensitivity to lateral movements of the membrane;the lateral movement is detected by capacitance measurement). FIG. 5Dshows that the enhanced designs can also be made both fingered andbalanced. The lateral sensing electrodes 512 f are symmetricallyarranged for sensing rotational acceleration (indicated by arrows 570).The quoted structures may adjust proof mass, for instance by addingsquashed balls from wire bonding, see FIG. 3), and spring constant sothat they can be tuned for different applications.

FIGS. 6, 7, and 8 show additional examples of MEMS device accelerometersto illustrate the variety of design options for devices with singlemember membranes integrated into substrate-based (620, 720, 820)packages with attached metal foil (610, 710, 810). The MEMS devices areencapsulated by molding compound (650, 750) in FIGS. 6 and 7, or by acan-type housing (850) in FIG. 8. The single member membrane design isdepicted in FIG. 6 in zero position (612 a) and activated (hovering)position (612 b). By lifting the proof mass from the substrate surfaceusing electrostatic force from the plate on the chip above thesubstrate, the lateral electrodes can also be made to electrostaticallyhover to detect vertical motion by restoring force signals; the forceneeded to keep the proof mass in place is proportional to theacceleration. Hovering membranes may be passivated when they are made ofa corrosive metal such as copper.

The symmetrically balanced design of a single membrane is shown in FIG.7 in activated (hovering) position (712 b). As stated above, theaddition of lateral sensing electrodes allows the sensing of lateralmovements of the membrane by capacitance measurements.

FIG. 8 illustrates a MEMS device with mounting chip 801 on one side onlyso that the smaller chip allows a membrane 812 with a sensor enlarged byproof mass 814. The sensitivity of the accelerometer is enhancedcompared to the arrangement discussed in FIG. 6. In general, MEMSdevices with the movable element integrated into substrate packages suchas shown in FIGS. 5A, 6, 7, and 8 lend themselves to multi-chip andstacked package-on-package applications.

Another embodiment of the invention is a process for fabricatingsubstrate based MEMS devices with the movable element integrated intothe device packages. For the process flow shown in FIGS. 9A to 9J, theprocess starts in FIG. 9A by selecting a flat substrate 920. Thesubstrate may be a sheet or board of a multi-metal layer plasticcomposite, a multi-metal layer ceramic composite, or a glass-fiberstrengthened board such as FR-4, with an example thickness between 70and 150 μm. Alternatively, substrate 920 may be a polymeric tape such asa polyimide-based tape. The substrate has a first surface 920 a and anopposite second surface 920 b. In the next process step, openings 922are formed through the thickness of the substrate, from the first to thesecond surface. One method of forming the openings is a punchingtechnique.

In FIG. 9B, a layer 930 of an adhesive material is attached to thesubstrate across the first surface 920 a, including across the openings922. The adhesive material is selected to stick to the substrate as wellas to metal. In FIG. 9C, a metal foil 910 is laminated across theadhesive layer on the first substrate surface. Suitable metals includecopper, nickel, and aluminum, and the thickness range typically is fromabout 10 to 25 μm, more generally from about 5 to 50 μm. The thicknessof the metal foil is selected so that the metal has the flexibility tooperate as the membrane of the MEMS device.

The patterning of metal layer 910 into a plurality of pads and asegment-to-become-membrane begins with depositing a photoresist layer960 across the metal foil on the flat first substrate surface andanother photoresist layer 961 across the contoured second substratesurface. As FIG. 9D shows, photoresist layer 961 follows the outline ofthe openings 922, where it also adheres to adhesive layer 930. Whilephotoresist layer 960 is masked, developed, and etched, photoresistlayer 961 remains unaffected. When metal layer 910 is etched andpatterned, photoresist layer 961 protects substrate surface 920 b.Finally, when photoresist layer 960 is removed, photoresist layer 961 isalso removed, and with it the portions of adhesive film 930, onto whichphotoresist layer 961 had adhered.

The result of the patterning steps for metal layer 910 is illustrated inFIG. 9E. The patterning created segment 912, which is to become themoving part of the MEMS device, for instance the membrane, and theplurality of pads 911. As membrane, segment 912 extends at leastpartially across opening 922. As a further result, the patterning ofmetal foil 910 creates interconnecting traces not shown in FIG. 9E.

In the step depicted in FIG. 9F, a semiconductor chip 901 withelectronic circuitry is flip-connected onto some of the metal pads sothat the chip extends at least partially across membrane 912 and opening922. The chip attachment is facilitated by bumps 970 made of solder orof gold. After the chip attachment, a gap 907 is formed between chip 901and membrane 912. Gap 907 is mostly determined by the height of bumps970.

In the next process step, illustrated in FIG. 9G, connecting metalbodies 940, such as solder balls, solder paste, or bodies of alow-melting gold alloy, are attached to a plurality of pads 911. Next,substrate 920 is prepared for the cuts along line 980, for instance by asaw, to singulate the substrate into discrete MEMS devices. Theseseparated units are depicted in FIG. 9H; each unit is an example MEMSdevice as described in FIG. 1.

Alternatively, before the cuts along line 980 in FIG. 9G areadministered, the process step shown in FIG. 9I may be performed. Theun-singulated substrate with the assembled chips is transformed into amore robust configuration by an encapsulation technology. In one processembodiment, the substrate with the assembled chips and the solder ballsattached is loaded into a mold for a transfer molding process. Theencapsulation material 950 is deposited over the features on substrateside 920 a so that cavity 922 remains open, or may, in some MEMSdevices, actually widen, as suggested in FIG. 9I. The encapsulationprocess may be a transfer molding technique, and the encapsulationmaterial may be an epoxy-based polymeric molding compound selected sothat the compound adheres strongly to substrate 920. As stated above,steel hillocks protruding into the mold cavity used for transfer moldingtechnology offer a low cost way to prevent a filling of opening 822 withcompound. After the molding step, the polymeric compound 950 is hardenedby polymerization, resulting in a sturdy package for the MEMS device.

Next, substrate 920 is prepared for the cuts along line 980, forinstance by a saw, to singulate the substrate into discrete MEMSdevices. These separated units are depicted in FIG. 9J; each unit issimilar to the example MEMS device described in FIG. 2.

While this invention has been described in reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. As an example, the invention applies to any material forthe MEMS device package, including plastics and ceramics, and thesemiconductor device, integrated circuits as well as discrete devices,including silicon, silicon germanium, gallium arsenide, or any othersemiconductor or compound material used in manufacturing.

As another example, the integration of the movable element into theleadframe-based package of a MEMS device can be applied topiezoresistive pressure sensors, where the conversion of pressure to anelectronically detectable signal relies on the elastic deformation of amembrane, or generally of a structure, that is exposed to the pressure.

As another example, the integration of the movable element into theleadframe-based package of a MEMS device can be applied to resonantpressure sensors, where the resonance frequency depends on themechanical stress in the vibrating microstructure.

As another example, the method of integrating the movable element intothe MEMS device package allows an inexpensive fine-tuning of themechanical transfer function by controlling the thickness of themembrane and by adding one or more mass units of squashed balls producedin wire bonding technique.

As another example, the integration of the movable element into thesubstrate-based package of a MEMS device can be applied topiezoresistive pressure sensors, where the conversion of pressure to anelectronically detectable signal relies on the elastic deformation of amembrane, or generally of a structure, that is exposed to the pressure.

As another example, the integration of the movable element into thesubstrate-based package of a MEMS device can be applied to resonantpressure sensors, where the resonance frequency depends on themechanical stress in the vibrating microstructure.

As another example, the method of integrating the movable element intothe substrate-based MEMS device package allows an inexpensivefine-tuning of the mechanical transfer function by controlling thethickness of the membrane and by adding one or more mass units ofsquashed balls produced in wire bonding technique.

It is therefore intended that the appended claims encompass any suchmodifications or embodiments.

What is claimed is:
 1. A MEMS device, comprising: a substrate having anopening extending between upper and lower surfaces; a first metal layerformed over the lower surface, the first metal layer defining adeflectable membrane and a pad, with the membrane extending at leastpartially across the opening; and an integrated circuit chip having anupper surface attached peripherally over the opening, through themembrane and pad to the lower surface of the substrate; wherein theintegrated circuit chip has the upper surface separated from anextending portion of the membrane by a gap and includes a second metallayer forming a fixed plate configured so that deflection of theextending portion of the membrane relative to the fixed plate can bemeasured as a change in capacitance.
 2. The device of claim 1, whereinthe integrated circuit chip is attached by solder to the membrane andpad.
 3. The device of claim 2, wherein the first metal layer furtherdefines a plurality of further pads laterally spaced outwardly from themembrane and pad, and the further pads are configured for connection ofthe device to external parts using solder bodies.
 4. The device of claim3, wherein the first metal layer comprises a patterned metal foilattached by adhesive to the lower surface of the substrate.
 5. Thedevice of claim 4, wherein the metal foil comprises at least one ofcopper, copper alloy, nickel, an iron-nickel alloy, or aluminum.
 6. Thedevice of claim 5, further comprising a tape formed over the lowersurface of the substrate; and wherein the metal foil is attached by theadhesive to the tape.
 7. The device of claim 6, wherein the opening isan upwardly and outwardly tapered opening.
 8. The device of claim 7,wherein the membrane is attached at one end to the tape, and is leftunattached at another end.
 9. The device of claim 1, wherein the firstmetal layer further defines a plurality of further pads laterally spacedoutwardly from the membrane and pad, and the further pads are configuredfor connection of the device to external parts using solder bodies. 10.The device of claim 1, wherein the first metal layer comprises apatterned metal foil attached by adhesive to the lower surface of thesubstrate.
 11. The device of claim 10, wherein the metal foil comprisesat least one of copper, copper alloy, nickel, an iron-nickel alloy, oraluminum.
 12. The device of claim 1, further comprising a tape formedover the lower surface of the substrate; and wherein the first metallayer comprises a patterned metal foil attached by the adhesive to thetape.
 13. The device of claim 12, wherein the membrane is attached atone end to the tape, and is left unattached at another end.
 14. Thedevice of claim 1, wherein the opening is an upwardly and outwardlytapered opening.
 15. A MEMS device, comprising: a substrate formed withan opening through the substrate from a first surface to an oppositesecond surface generally coplanar with the first surface; a metal foillayer adhered over the substrate first surface and patterned into aplurality of pads and a segment extending at least partially across theopening; and a semiconductor chip having electronic circuitry, the chipbeing flip-connected onto the pads so that the chip spans at leastpartially across the opening, separated from the segment by a gap. 16.The device of claim 15 further including a polymeric compound depositedand hardened over the device, preserving the opening in the substrate.